Introduce new transparent bars sysui flags.

Two new public sysui flags for views to request transparent
bars:
  View.SYSTEM_UI_FLAG_TRANSPARENT_STATUS
  View.SYSTEM_UI_FLAG_TRANSPARENT_NAVIGATION

This first change simply introduces the flags at the framework
level, and makes the requisite layout tweaks to WM.

As part of this change, expunge the term "hideybars" from the
codebase. The flag to declare support for transient bars is called:
  View.SYSTEM_UI_FLAG_ALLOW_TRANSIENT

Final visuals/transitions between opaque/transparent bars will be
done as a subsequent change. Right now the transparent style is
identical to the transient bars.

Change-Id: I5ead9c5e7b77f212df5b2a5f6e770596cd2226f3